Method for suspending wells

ABSTRACT

A process for killing and suspending mixtures of hydrocarbonaceous fluid production, particularly oil, in a formation containing same which minimizes formation damage. First, a &#34;Spacer volume&#34; of liquid containing a chemical blowing agent is directed into the formation&#39;s productive interval. Said blowing agent decomposes in the formation and generates gas sufficient to force hydrocarbonaceous fluids away from a wellbore in said formation. Afterwards, a solidifiable pumpable gel mixture is placed via a wellbore into the formation&#39;s productive interval and also within said wellbore. Said mixture solidifies in the formation and forms a gel plug within the wellbore. Thereafter, a light weight cement is placed over said gel plug effectively &#34;killing&#34; and suspending the production of hydrocarbonaceous fluids. The gel mixture comprises (1) water; (2) a cross linkable polymer having at least one functional group selected from a member of the group consisting of an amide, an amine, a hydroxyl, or a thiol group; and (3) a partially methylated aminoplast resin which cross links with said polymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Application Ser.No.890,679, filed July 30, 1986, now U.S. Pat. No. 4,817,719 and is relatedto Application Ser. No. 917,324, filed Oct. 9, 1986, now U.S. Pat. No.4,834,180 and Application Ser. No. 138,176, filed Dec. 28, 1987, nowU.S. Pat. No. 4,813,484.

FIELD OF THE INVENTION

This invention is directed to a method for minimizing formation damagewhen a well is "killed" and production is suspended.

BACKGROUND OF THE INVENTION

When productive intervals are completed in exploratory wells, it iscommon practice to "suspend" wells for a period of time to allowconstruction of pipelines and gathering facilities in conjunction withfield development. In remote locations (offshore, e.g.), it may even benecessary to plug and abandon zones found productive until developmentwells can be drilled.

In many cases, it is not known how long a well may be suspended.Therefore, thorough measures are taken to isolate the productiveinterval from the surface. For example, following a flow test of thezone indicating productivity at commercial rates, the zone will be"killed" with completion fluid, a bridge plug set above the perforatedinterval, and a cement plug placed on top of the bridge plug. Two orthree additional bridge plug/cement plug combinations may be placedabove the interval to insure zone isolation during suspension.

When the suspended well is re-entered to open the interval toproduction, the plugs must be drilled out. Frequently this results insubstantial loss of drilling fluid to the zone. This is especially trueif the zone was stimulated (fracturing or acidizing, e.g.) as part ofthe initial well test program. Once the plugs are drilled out, the zoneis opened to production. Usually, a lower rate than the initial testrate is observed because of damage which occurred during re-entry.

Therefore, what is needed is a method which will allow a producing wellto be "killed" and suspended without causing undue formation damage.Utilization of said method would result in maintaining the producingrate near that initially determined. Well re-entry costs would bereduced, and formation damage minimized.

SUMMARY OF THE INVENTION

This invention is directed to a method for "killing" and suspending oilproduction in a well which results in a reduction in formation damage.In the practice of this method, a "spacer volume" of liquid containing asurfactant is directed into the formation's productive interval.Surfactant contained in said liquid causes the formation to become morereceptive to a solidifiable gel solution. Said chemical blowing agentdecomposes and generates a gas which blocks formation pores and excludesformation fluids. Thereafter, a pumpable solidifiable gel mixture isplaced into the wellbore substantially at the formation's productiveinterval. The gel mixture which is utilized herein comprises (a) water;(2) a cross linkable polymer having at least one functional groupselected from a member of the group consisting of an amine, an amide, ahydroxyl, or a thiol group; and (3) a partially methylated aminoplastresin which cross links with said polymer.

Subsequently, the gel mixture solidifies after entering said productiveinterval of the formation while causing a solid gel plug to form in thewellbore substantially in the area of the wellbore's productiveinterval. Said solidified gel mixture within the formation and thewellbore is sufficient to withstand environmental conditions in theformation depths, including pressures. As a result of the solidified gelmixture in the formation and gel plug within the wellbore, oil and otherhydrocarbonaceous fluids cease to flow from the formation into saidwellbore, thus "killing" the well.

Thereafter, the depth of the top of the solid gel plug is determined sothat a desired amount of light (low density) concrete can be placed oversaid gel plug. Next, a desired amount of a light concrete is placed oversaid gel plug in an amount sufficient to suspend the production ofhydrocarbonaceous fluids from said formation to the surface.

It is therefore an object of this invention to eliminate the need for akill or completion fluid when "killing" a well.

It is another object of this invention to eliminate the need for abridge plug when well production is suspended.

It is yet another object of this invention to minimize formation damageresultant from loss of drilling fluid which in prior art methods oftenentered the productive interval of a formation thereby decreasing theformation's permeability.

It is a yet further object of this invention to reduce well re-entrycosts upon termination of the well suspension period.

It is a still yet further object of this invention to maintain theproduction rate substantially near the initial production rate upontermination of the well suspension period.

It is a still yet further object of this invention to pump the requiredmaterials into the wellbore without the need to circulate them into andout of the wellbore as was required in prior art methods.

It is even yet another object of this invention to provide a gelationreaction which can proceed under all pH conditions encountered in ahydrocarbonaceous reservoir.

It is even then yet a further object of this invention to provide for asubstantially stable gel when high temperatures are encountered in areservoir.

It is even then yet a still further object of this invention to providefor a gelation reaction which will proceed in a saline hydrocarbonaceousreservoir environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic representation of a prior art method for "killing"and suspending oil production in a well.

FIG. 2 is a schematic representation of the method which is disclosedherein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

When "killing" a productive interval of a well, as shown in FIG. 1, akill fluid 14 is generally injected in wellbore 12. Said kill fluidenters formation 10 via perforations 16 "drowning" the productiveinterval in formation 10. Thereafter, a bridge plug 18, which isgenerally of a metallic construction, is caused to bind securely againstwellbore 12. Once bridge plug 18 is securely bound, a cement plug 20hardens and with the bridge plug causes the productive interval ofwellbore 12 to be closed to the surface and thereby suspends oilproduction into said wellbore. To more effectively secure the productiveinterval against oil production, multiple cement plug/bridge plugcombinations can be utilized in wellbore 12 as is shown in FIG. 1.

In the practice of one embodiment of this invention, a spacer volume ofa liquid containing a surfactant and a chemical blowing agent is pumpeddirectly into the wellhead. This spacer volume of liquid is used toclear the formation and well of materials which might interfere withadherence and solidification of a gel mixture. Surfactant contained insaid liquid causes the formation to become more receptive to asolidifiable gel solution. A surfactant enhances the displacementefficiency of hydrocarbons by lowering interfacial tension. A surfactantalso alters formation wettability to achieve better bonding of gel torock. Generally, about two wellbore tubing volumes of fluid will besufficient to clear and condition said formation and wellbore. Thespacer volume of liquid comprises an aqueous medium into which is placeda surfactant and a decomposable chemical blowing agent. When utilized inthe spacer volume of liquid the surfactant can stabilize a foam or canassist in dispersing a blowing agent. Said aqueous medium can comprisefresh water, formation brine, sea water, brackish water, and mixturesthereof. Placement of the spacer volume of liquid into the formationalso substantially inhibits production of hydrocarbonaceous fluids whenperforming the subsequent steps of this invention.

Foam-forming surfactants suitable for use in the present invention cancomprise substantially any which are capable of being dissolved ordispersed in an aqueous liquid solution containing a nitrogen containingcompound and any added activator or inhibitor which remainssubstantially inert during a nitrogen-gas-producing reaction of thenitrogen containing compounds. Examples of suitable surfactants comprisenonionic and anionic surfactants, e.g., Siponate DS-10 available fomAmerican Alcolac Company, mixtures of the Siponate or similar sulfonatesurfactants with sulfated polyoxyalkylated alcohol surfactants, e.g.,the NEODOL sulfate surfactants available from Shell Chemical Company;sulfonate sulfate surfactant mixtures, e.g., those described in the J.Reisberg, G. Smith and J. P. Lawson U.S. Pat. No. 3,508,612; petroleumsulfonates available from Bray Chemical Company; Bryton sulfonatesavailable from Bryton Chemical Company; Petronates and Pyronatesavailable from Sonnoborn Division of Witco Chemical Company; fatty acidand tall oil acid soaps, e.g., Actynol Heads from Arizona ChemicalCompany; nonionic surfactants, e.g., Triton X100; and the likesurfactant materials which are soluble or dispersible in aqueousliquids. These surfactants are disclosed in RE 30,935 which issued toRichardson et al. on May 18, 1982. This patent is incorporated herein byreference.

Chemical blowing agents which can be utilized herein includedinitrosopentamethylenetetramine (DNPT), blends of sodium hydrogencarbonate and nitrogen releasing agents such as p-toluene sulfonylhydrazide, and p,p'-oxybis (benzenesulfonyl hydrazide). Other chemicalblowing agents which can be utilized include azodicarbonamide, andalkali metal salts of azodicarboxylic acid.

DNPT and sodium hydrogen carbonate can be used in conjunction with saidsurfactant in an aqueous medium. Since DNPT is only slightly soluble incold water, warm water is required to achieve significant watersolubility. Solutions of 0.5% by weight can be obtained at ambienttemperature. Solubilities in excess of 1% by weight are achieved above40° C. Warm water can be obtained by preheating water to be injected orby reinjecting warm produced water. Enhancement of the low temperaturesolubility of DNPT can be obtained by the use of chemicals. Saidchemicals include dimethylformamide (DMF) and dimethylsulfoxide (DMSO).Should it be desired to accelerate the decomposition of DNPT in-situ,weak acids such as ammonium sulfate or acetic anhydride can be added tothe formulation. As will be understood by those skilled in the art, theamount of chemical utilized will depend upon such factors as the amountand temperature of water utilized, chemical composition of the water,the amount of DNPT utilized, and the reservoir pressure.

Although sodium hydrogen carbonate and other bicarbonate foaming agentscan be utilized, they are limited by an equilibrium which reduces yieldwith increasing pressure. To overcome this limitation, bicarbonatedecomposition can be pH driven with formulations containing suitablecompounds for pH depression with temperature increase. One such compoundis the nitrogen releasing blowing agent, p-toluene sulfonyl hydrazide.Bicarbonate decomposition generates carbon dioxide. The addition of asuitable amount of p-toluene sulfonyl hydrazide, which liberates watersoluble, acidic byproducts upon decomposition, causes substantiallyincreased volumes of carbon dioxide to be released from solution due tobicarbonate decomposition.

Azodicarbonamide similar to DNPT is soluble in water only at elevatedtemperatures. Since azodicarbonamide is available in powder form withaverage particle size in the micron range, solid dispersions can beutilized. A disperson can be made with adequate mixing by placing micronsized azodicarbonamide in a solution containing a suitable surfactant.The amount of azodicarbonamide should be sufficient to create the volumeof gas required to obtain a fluid diversion effect. One such class ofsurfactants is alkyl naphthalene sulfonates, which can be purchased fromGAF as the Nekal series, located in New York. Should it be desired toaccelerate the decomposition of azodicarbonamide, an alkali carbonatecan be utilized to modify decomposition kinetics. Alkali carbonateswhich can be utilized include sodium carbonate and potassium carbonate.Thus, azodicarbonamide will prove to have enhanced potential for use incarbonate reservoirs. Azodicarbonamide can be included in amicroemulsion for injection into the formation. A method for making amicroemulsion is disclosed in U.S. Pat. No. 4,008,769 which issued toChang on Feb. 22, 1977. This patent is incorporated by reference herein.

Where alkaline conditions are acceptable or desirable, the sodium saltof azodicarboxylic acid can be used as a chemical blowing agent. Thisblowing agent can be formed on site by the treatment of azodicarbonamidewith sodium hydroxide and an alkali carbonate with resulting ammoniaevolution. When heated, this extremely water soluble salt liberatesnitrogen and carbon dioxide, yet it is very stable at room temperaturein basic solutions having a pH greater than 12. Surfactant addition canbe reduced or even eliminated in some instances due to surfactantproduction in-situ. The pH decline from hydroxide consumption willaccelerate the foam decomposition reaction. Toluene sulfonyl hydrazideand p,p'-oxybis(benzenesulfonyl hydrazide) also develop water solubilityat high pH, but the modified azodicarbonamide is preferred.

In another embodiment of this invention, any one of the chemical blowingagents above mentioned can be incorporated into an aqueous medium forutilization as said spacer volume of liquid. The amount of blowing agentutilized will vary from about 0.05% to about 0.5% by weight. The gasgenerated upon decomposition will be sufficient to form gas asindividual bubbles but insufficient to cause foam to form at typicalreservoir pressures. Since the carrier fluid for the chemical blowingagent is an aqueous medium, bubbles which nucleate will do so inpreferential pathways for water flow. Pore blocking and fluid diversionwill result in localized increase in flow resistance. Since thedecomposing chemical agent releases a gas which causes bubbles to formand block pores within a more permeable zone of the formation, asubsequently injected solidifiable gel will be directed to a lesspermeable zone in the formation thereby minimizing the amount of gelused and facilitating the flow of hydrocarbonaceous fluids into the wellonce production is commenced. If it is desired to create additional gas,the amount of chemical blowing agent can be increased.

When it is desired to form a foam, a chemical blowing agent and asurfactant are added into an aqueous medium in an amount sufficient tocreate a foam at reservoir conditions. The amount of chemical blowingagent utilized will be from about 0.51% to about 5.0% by weight. Theamount of surfactant utilized will be an amount sufficient for foamstabilization and will generally be from about 0.1% to about 2% byweight. After mixing the blowing agent and surfactant together in anaqueous medium, a spacer volume of the aqueous medium is injected intothe formation. Once the medium is within the formation, the chemicalblowing agent decomposes thereby liberating sufficient gas to create afoam. The foam which is generated is sufficient to close pores in a morepermeable zone of a formation. A solidifiable gel is thereafter injectedinto the formation and is then diverted to a less permeable zone.Placement of the spacer volume of liquid into the formationsubstantially inhibits the production of hydrocarbonaceous fluids fromthe formation during the subsequent steps of this invention.

As is known to those skilled in the art, the pumping or injection ratesutilized should be maintained below those rates which will create apressure sufficient to fracture the formation.

In both embodiments, the chemical blowing agent is selected on the basisof reservoir temperature, mineralogy, depth, and environmentalconditions. As required, pH buffers, accelerators, or inhibitors can beincorporated into the spacer volume of aqueous liquid prior to injectioninto the formation or reservoir. Choice of accelerators or inhibitorswill be specific to the selected blowing agent. Accelerators which canbe used for azodicarbonamide include alkali carbonates, basic metalsalts of lead, cadmium, or zinc such as dibasic lead phthalate, andpolyols such as glycols and glycerol. Inhibitors which can be utilizedinclude barium salts and neutral pH buffers. Accelerators which can beused for DNPT include mineral acids and salts of mineral acids such aszinc chloride. Stabilizers which can be used for DNPT include oxides,hydroxides, or carbonates of calcium, barium, zinc, or magnesium. Thesize of the spacer volume of liquid will depend upon the extent of theprescribed treatment area. The injection rate of said spacer volume ofliquid should be sufficient to allow fluid placement into the zone orzones desired to be treated prior to significant gas release. Bubbles orfoam generated in a high permeability zone will lead to flow diversionand enhanced sweep of the formation or reservoir prior to injection of asolidifiable gel. Upon incorporation of these compounds in the spacervolume of liquid which is injected via wellbore 12 into formation 10,said compounds have the ability to minimize fluid retention, andcondition the formation for greater receptivity of a solidifiablepumpable gel mixture.

Afterwards, as is shown in FIG. 2, a pumpable solidifiable gel mixtureis directed into wellbore 12, usually by pumping it into the wellhead.Said gel mixture is allowed to flow down wellbore 12 in formation 10until it comes in contact with the productive interval of formation 10.The injection or pumping rates should be maintained below thosepressures which will cause the formation to fracture. At the productiveinterval, said gel mixture enters the productive interval of formation10 via perforations 16. Sufficient solidifiable gel is allowed to entersaid productive interval thereby closing off said interval to productionof hydrocarbonaceous fluid mixtures, particularly oil. Additionalsolidifiable gel material is allowed to enter wellbore 12 which materialcontacts said productive interval until said gel has filled wellbore 12above the productive interval.

The solidifiable gel mixture subsequently forms a solid gel plug 24within wellbore 12. It also forms a solid formation gel 22 in theproductive interval of formation 10. Gel plug 24, upon solidification,is of a composition and strength sufficient to support a cement plug 20thereabove in addition to forming an impermeable barrier to the flow offluids from the productive interval. Cement plug 20 is comprised on alight cement. Representative cements are foamed cements and a lightweight (low density) cement sold by Haliburton under the Spherelitetradename. Similar cement compositions are disclosed in U.S. Pat. Nos.3,902,911 and 4,120,360 which are hereby incorporated by reference. Uponhardening, the cement forms a further impermeable barrier to the flow offluids from the productive interval of formation 10. As will beunderstood by those skilled in the art, quantities of "spacer" volumesof liquid 26, said gel mixture, and a light weight cement will varydepending on formation parameters encountered. Also, a solidified gelplug, and a cement plug can be alternated in wellbore 12 until asufficient number have been placed depending on the productive intervaldepth and length of the suspension period.

After the gel plug and light-weight cement plug have set, a pressuretest is applied to the plug combination to insure competency and thedesired plugging effect. Upon confirmation of the competency andplugging effect, the distance from the wellhead to the finallight-weight cement plug is determined for re-entry at a later date.This determination can be made by running a dummy or wireline intowellbore 12. When it is desired to commence production ofhydrocarbonaceous fluids from said formation, cement plug 20 is drilledout and gel plug 24 caused to be removed. Said removal can beaccomplished by mechanical or chemical means. Since the productiveinterval of formation 10 is closed from wellbore 12 by the solidifiedgel, any pressurized drilling fluid utilized to remove cement plug 20from wellbore 12 will not enter the productive interval and damage it.

Since a kill fluid is not utilized in this invention, hydrocarbonaceousfluids, particularly oil, are produced through said productive intervalat substantially the initial production rate once production resumes.Re-entry costs are substantially reduced since more costly drill bitsare not required as a metallic bridge plug is not used.

One method of making a suitable pumpable mixture is discussed in U.S.Pat. No. 4,333,461 issued to Muller on June 8, 1982 which is herebyincorporated by reference. The stability and rigidity of gel plug 24will depend upon the physical and chemical characteristics of the gelplug. As is known to those skilled in the art, gel plug 24 should be ofa stability and rigidity which will withstand the weight of cement plug20 and environmental well conditions.

Often, it will be necessary to increase the density of the pumpable gelto obtain the desired stability and rigidity therein. To accomplishthis, a solid non-reacting material can be added to the pumpable gelmixture. Preferred non-reacting solid materials include solid rock salt,calcium carbonate, and suitably crushed mollusk shells, such as oystershells.

Other gel mixtures can be used to obtain a desired stability andrigidity. A preferred mixture used to obtain the desired stability andrigidity, for example, is a polymer having functional groups such asNH₂, --CONH₂, --OH, --SH which can be gelled with methylated MF resins.Some acceptable polymers include polyacrylamide, Kelco's S-130biopolymer, acrylamide modified polyvinyl alcohol ("AMPVA"), Xanthanbiopolymers, poly (acrylamide-co-acryl-amido-2-methyl-propanesulfonate)"AM-AMPS", "Phillips HE" polymers (a family of acrylamide containingcopolymers), and polyvinyl alcohol. "Phillips HE" polymer comprisescopolymers of N-vinyl-2-pyrrolidone (VP) and acrylamide (AM) in whichthe weight ratios of VP:AM preferably range from about 30:70 to about70:30. Said polymer is discussed in U.S. Pat. No. 4,644,020 which issuedto Stahl on Feb. 17, 1987. This patent is hereby incorporated byreference herein. Polymers mentioned in U.S. Pat. No. 4,157,322, supra,may be utilized as long as those polymers contain the functional groupsabove mentioned. Polymer concentration in said gels range from about 0.1to about 5.0 wt. percent. These polymer concentrations vary dependingupon the molecular weight of polymer used. Lower molecular weightpolymers require higher polymer concentration to gel. A polymerconcentration of about 0.2-5.0 wt. percent is preferred. Thiscrosslinking/co-gelation method produces high integrity polymer gelsable to withstand high temperatures and high salinity conditions oftenfound in subterranean hydrocarbonaceous formations.

Methylated MF derived as a reaction product of melamine and formaldehydehas a molar ratio of between 1-6. A ratio of 3-6 is commonly found incommercial resins. The methylol group, --CH₂ OH and its methylatedvarieties are reactive to various functional groups such as NH₂, --CONH₂--OH, --SH and can also self-condense to form cured resins. Itspreparation is convenient and well documented in preparative polymermanuals.

The melamine resin that is utilized in this invention can be acommercial product such as Cyanamid's Parez® resins. Included amongthese melamine-formaldehyde (melamine) resins which are useful in thisinvention are the partially methylated resins and the hexamethoxymethylresins (i.e., American Cyanamid's Parez, Cymel®373, Cymel 370, Cymel303, and Cymel 380). The resin, however, has to be one that is solubleor dispersible in an aqueous medium. Other amino resins can also beused. Non-limiting examples are urea-formaldehyde, ethylene andpropylene urea formaldehyde, trizone, uran, and glyoxal resins. Theamount of MF resins required for adequate gel formation is in the ratioof 10:1-1:10 polymer to amino resins. Preferred polymer concentrationsare from about 0.2 to about 5.0 wt. percent. Amino resins are preferredcrosslinkers because they (1) are economical to use: (2) can be appliedto a wide variety of polymers; (3) form thermally stable, brine tolerantgels; and (4) do not need an acid or base catalyst.

The gelation rate of the composition depends on the amount of each ofthe components and the temperature at which the reaction is conducted.Thus, one can tailor the gel rate and gel strength of the composition byadjusting the amount of polymer, the resin amount and the temperature.The higher the temperature at given concentrations of resin and polymer,the faster the gelation time. If a thicker gelled composition isdesired, the polymer and resin concentrations may be increased for agiven temperature.

Gels resultant from the gelation reaction were formed in about a 15 to30 wt. % brine solution containing at least about 1500 ppm Ca(II) and atleast 500 ppm Mg (II). Said formed gels were stable as determined bysustained gel integrity and low gel shrinkage at least about 195° F.,for at least three months. Examples of preferred gel compositions areset forth below.

    ______________________________________                                        Gelation of Melamine - Formaldehyde Crosslinker                                                      30%     Deionized                                                                             Parez                                  Example Polymer        Brine.sup.8                                                                           Water   613.sup.1                              ______________________________________                                                10% AMPVA.sup.2                                                       #1      5 g            5 g     0       0.4 g                                  #2      2.5 g          5 g     2.5 g   0.4 g                                          AMPS-AMPVA.sup.3 10%                                                  #3      2.5 g          5 g     2.5 g   0.4 g                                  #4      5 g            5 g     0       0.4 g                                          PVA.sup.4 5%                                                          #5      5 g            2.5 g   2.5 g   0.4 g                                          AMPS-PVA.sup.5 10%                                                    #6      5 g            2.5 g   2.5 g   0.4 g                                          Magnifloc.sup.6 1%                                                    #7      5 g            5 g     0       0.4 g                                  #8      5 g            2.5 g   2.5 g   0.4 g                                          AM-AMPS.sup.7 1%                                                      #9      5 g            5 g     0       0.4 g                                  #10     2.5 g          5 g     2.5 g   0.4 g                                  ______________________________________                                        Gelation with Trimethylolmelamine (TM)                                                               30%     Deionized                                      Example Polymer        Brine.sup.8                                                                           Water   TM                                     ______________________________________                                                S-130 1%.sup.9                                                        #11     5 g            5 g     --      0.4 g                                  #12     5 g            5 g     --      0.2 g                                  #13     2.5 g          5 g     2.5 g   0.4 g                                  #14     2.5 g          5 g     2.5 g   0.4 g                                          HE E 2%                                                               #15     2.5 g          5 g     2.5 g   0.4 g                                  #16     2.5 g          5 g     2.5 g   0.2 g                                          Xanthan.sup.11 2%                                                     #17     2.5 g          5 g     2.5 g   0.4 g                                  #18     2.5 g          5 g     2.5 g   0.2 g                                  ______________________________________                                         .sup.1 A commercial 80% active amino resin obtainable fom American            Cyanamid                                                                      .sup.2 Acrylamide modified polyvinyl alcohol                                  .sup.3 Acrylamido2-methyl-propanesulfonate/acrylamide modified polyvinyl      alcohol                                                                       .sup.4 Polyvinyl alcohol                                                      .sup.5 Acrylamido2-methyl-propanesulfonate/polyvinyl alcohol                  .sup.6 Polyacrylamide obtained from American Cyanamid                         .sup.7 Poly (acrylamideco-acrylamido-2-methyl-propanesulfonate)               .sup.8 30% NaCl, 2000 ppm Ca, 1000 ppm Mg                                     .sup.9 Kelco "S130" biopolymer                                                .sup.10 Phillips HE                                                           .sup.11 Pfizer Flocon biopolymer                                         

Cement plug 20 can be removed from wellbore 12 by drilling. However,formation gel 22 and gel plug 24 can also be removed in several otherways. Several variations are provided for. One variation, which can beutilized to facilitate removal of the gel plug 24 from wellbore 12 andformation gel 22 is to form a solid gel plug 24 or formation gel 22containing a gel breaker. This gel breaker, included in the gel mixture,is selected from a group of chemical compounds which can break down thesolid gel to temperatures of less than from about 60° F. to about 250°F. Generally, this breakdown will occur within from about 2 hours toabout 24 hours depending upon type and concentration of breaker added.Chemicals satisfactory for use as gel breaker, and which areincorporated into the gel mixture, include enzymes and oxidizing agents(such as sodium persulfate) suitable for breaking down the solid gel.Other gel breakers sufficient for this purpose are discussed in U.S.Pat. No. 4,265,311 issued to Ely on May 5, 1981, which patent is herebyincorporated by reference. These chemicals are readily available fromchemical suppliers and with the exception of enzyme breakers are soldunder their chemical names. Enzyme breakers can be obtained from oilfield service companies. The concentration of gel breaker incorporatedinto the gel mixture will vary from about 0.01 weight percent to about0.10 weight percent, preferably about 0.05 weight percent of the gelmixture. Upon cooling to a temperature of from about 60° F. to about150° F., the gel breaker will breakdown the solid gel causing it toliquify which will facilitate removal of gel plug 24 and formation gel22.

Another method for breaking the gel is to contact the solidified gelwith a mineral acid after a suitable or desired time interval. In thoseinstances where it is undesirable to have a gel breaker incorporatedinto the gel mixture to remove solid gel plug 24 or formation gel 22, itis preferred to use hydrochloric acid of a strength sufficient tosolubilize the solid gel plug 24 and formation gel 22 without attackingformation 10. Hydrochloric acid, and acids similar thereto, can be usedto breakdown the solid gel on contact. Hydrochloric acid of aconcentration of about 10 percent to about 28 percent preferably about15 percent, by volume of solution, will generally be sufficient for thispurpose. Although hydrochloric acid has been mentioned, other similarmineral acids and strong organic acids may be employed depending upontheir availability, as is known to those skilled in the art.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

We claim:
 1. A method for shutting in a well which minimizes formationdamage comprising:(a) placing into said formation a solution containingwater and a chemical blowing agent; (b) causing said chemical blowingagent to decompose thereby liberating a gas which forces formationfluids away from a wellbore into said formation; (c) pumping asolidifiable gel mixture into the productive interval of the formationvia a wellbore where said mixture comprises(i) water, (ii) 0.2 to 5.0wt. percent of a cross linkable polymer having at least one functionalgroup selected from a member of the group consisting of an amine, anamide, a hydroxyl, or a thiol group, and (iii) 0.02 to 50.0 wt. percentof a partially methylated aminoplast resin which cross links with saidpolymer; (d) causing said gel mixture to become a solid thereby forminga gel plug within said wellbore and a formation gel within saidformation sufficient to withstand environmental formation conditions andpressures which cause hydrocarbonaceous fluids to flow into saidwellbore from the formation areas near said wellbore; (e) placing insaid wellbore on top of said plug an amount of cement sufficient toisolate said productive interval when said cement hardens while beingsupported by said plug; and (f) allowing said cement to set which setcement in combination with said gel plug is competent to excludeproduction fluids from said wellbore.
 2. The method as recited in claim1 wherein said resin is a member selected from the group consisting ofmelamine-formaldehyde, urea formaldehyde, ethylene urea formaldehyde,propylene urea formaldehyde, triazone, uran, and glyoxal.
 3. The methodas recited in claim 1 where said polymer is a member selected from thegroup consisting of polyacrylamide, polyvinyl alcohol, Xanthanbiopolymers, Kelco S-130 biopolymer, poly(acrylamide-co-acrylamido-2-methyl-propanesulfonate), Phillips HEpolymers, and acrylamide modified polyvinyl alcohol.
 4. The method asrecited in claim 1 wherein the ratio of polymer to said resin requiredfor gelation is from about 10:1 to about 1:10.
 5. The method as recitedin claim 1 where said chemical blowing agent is a member selected fromthe group consisting of dinitrosopentamethylenetetramine,azodicarbonamide, and p,p'-oxybis(benzene sulfonyl hydraide).
 6. Themethod as recited in claim 1 where said chemical blowing agent isazodicarbonamide where decomposition is accelerated by alkalicarbonates.
 7. The method as recited in claim 1 where said chemicalblowing agent is an alkali metal salt of azodicarboxylic acid which upondecomposition liberates nitrogen and carbon dioxide gases.
 8. The methodas recited in claim 1 where said liquid solution is injected into saidformation by at least one injection well.
 9. The method as recited inclaim 1 where said liquid solution contains therein a pH adjustor, anaccelerator, or an inhibitor sufficient to provide for variablepropagation distances within said formation prior to gas generation. 10.The method as recited in claim 1 where said chemical blowing agentcomprises sodium hydrogen carbonate and p-toluene sulfonyl hydrazidewhich decompose to release nitrogen and carbon dioxide gases.
 11. Themethod as recited in claim 1 where said blowing agent is contained insaid liquid solution in from about 0.05% to about 5.0% by weight.
 12. Amethod for shutting in a well which minimizes damage in a formationcomprising:(a) placing into said formation a solution containing water,a surfactant, and a chemical blowing agent; (b) causing said chemicalblowing agent to decompose thereby forming a foam with said surfactantwhich foam establishes fluid flow, minimizes fluid retention, and forcesformation fluids away from a wellbore in said formation; (c) pumping asolidifiable gel mixture into the productive interval of said formationvia a wellbore where said mixture comprises(i) water, (ii) 0.2 to 5.0wt. percent of a cross linkable polymer having at least one functionalgroup selected from a member of the group consisting of an amine, anamide, a hydroxyl, or a thiol group, and (iii) 0.02 to 50.0 wt. percentof a partially methylated aminoplast resin which cross links with saidpolymer; (d) causing said gel mixture to become a solid, thereby forminga gel plug within said wellbore and a formation gel within saidformation sufficient to withstand environmental formation conditions andpressures which cause hydrocarbonaceous fluids to flow into saidwellbore from the formation areas near said wellbore; (e) placing insaid wellbore on top of said plug an amount of cement sufficient toisolate said productive interval when said cement hardens while beingsupported by said plug; (f) allowing said cement to set which set cementin combination with said gel plug is competent to exclude productivefluids from said wellbore; (g) causing said solid formation gel, saidgel plug, and said hardened cement to be removed after the well has beenshut in for a desired time interval; and (h) producing thereafterhydrocarbonaceous fluids from said formation via said wellbore.
 13. Themethod as recited in claim 12 wherein said resin is a member selectedfrom the group consisting of melamine-formaldehyde, urea formaldehyde,ethylene urea formaldehyde, propylene urea formaldehyde, triazone, uran,and glyoxal.
 14. The method as recited in claim 12 where said polymer isa member selected from the group consisting of polyacrylamide, polyvinylalcohol, Xanthan biopolymers, Kelco S-130 biopolymer, poly(acrylamide-co-acrylamido-2-methyl-propanesulfonate), Phillips HEpolymers, and acrylamide modified polyvinyl alcohol.
 15. The method asrecited in claim 12 wherein the ratio of polymer to said resin requiredfor gelation is from about 10:1 to about 1:10.
 16. The method as recitedin claim 12 where said chemical blowing agent is a member selected fromthe group consisting of dinitrosopentamethylenetetramine,azodicarbonamide, p,p'-oxybis(benzenesulfonyl hydrazide).
 17. The methodas recited in claim 12 where said chemical blowing agent isazodicarbonamide where decomposition is accelerated by alkalicarbonates.
 18. The method as recited in claim 12 where said chemicalblowing agent is an alkali metal salt of azodicarboxylic acid which upondecomposition liberates nitrogen and carbon dioxide gases.
 19. Themethod as recited in claim 12 where said liquid solution is injectedinto said formation by at least one injection well.
 20. The method asrecited in claim 12 where said liquid solution contains therein a pHadjustor, an accelerator, or an inhibitor sufficient to provide forvariable propagation distances within said formation prior to gasgeneration.
 21. The method as recited in claim 12 where said chemicalblowing agent comprises sodium hydrogen carbonate and p-toluene sulfonylhydrazide and carbon dioxide gases.
 22. The method as recited in claim12 where said blowing agent is contained in said liquid solution in fromabout 0.05% to about 5.0% by weight.
 23. A method for shutting in a wellwhich minimizes damage to a formation comprising:(a) placing into saidformation a solution containing water and a chemical blowing agent andwhich agent is a member selected from the group consisting of sodiumhydrogen carbonate and p-toluene sulfonyl hydrazide, an alkali metalsalt of azodicarboxylic acid, azodicarbonamide,dinitrosopentamethylenetetramine, and p,p'-oxybis(benzenesulfonylhydrazide); (b) causing said chemical blowing agent to decompose therebyliberating a gas which forces formation fluids away from a wellbore insaid formation; (c) pumping a solidifiable gel mixture into theproductive interval of said formation via a wellbore where said mixturecomprises(i) water, (ii) 0.2 to 5.0 wt. percent of a cross linkablepolymer having at least one functional group selected from a member ofthe group consisting of an amine, an amide, a hydroxyl, or a thiolgroup, and (iii) 0.02 to 50.0 wt. percent of a partially methylatedaminoplast resin which cross links with said polymer; (d) causing saidgel mixture to become a solid, thereby forming a gel plug within saidwellbore and a formation gel within said formation sufficient towithstand environmental formation conditions and pressures which causehydrocarbonaceous fluids to flow into said wellbore from the formationareas near said wellbore; (e) placing in said wellbore on top of saidplug an amount of cement sufficient to isolate said productive intervalwhen said cement hardens while being supported by said plug; (f)allowing said cement to set which set cement in combination with saidgel plug is competent to exclude productive fluids from said wellbore;p1 (g) causing said solid formation gel, said gel plug, and saidhardened cement to be removed after the well has been shut in for adesired time interval; and (h) producing thereafter hydrocarbonaceousfluids from said formation via said wellbore.
 24. The method as recitedin claim 23 wherein said resin is a member selected from the groupconsisting of melamine-formaldehyde, urea formaldehyde, ethylene ureaformaldehyde, propylene urea formaldehyde, triazone, uran, and glyoxal.25. The method as recited in claim 23 where said polymer is a memberselected from the group consisting of polyacrylamide, polyvinyl alcohol,Xanthan biopolymers, Kelco S-130 biopolymer, poly(acrylamide-co-acrylamido-2-methyl-propanesulfonate), Phillips HEpolymers, and acrylamide modified polyvinyl alcohol.
 26. The method asrecited in claim 23 where the ratio of polymer to said resin requiredfor gelation is from about 10:1 to about 1:10.
 27. The method as recitedin claim 23 where said chemical blowing agent is azodicarbonamide wheredecomposition is accelerated by alkali carbonates.
 28. The method asrecited in claim 23 where said liquid solution is injected into saidformation by at least one injection well.
 29. The method as recited inclaim 23 where said liquid solution contains therein a pH adjustor, anaccelerator or an inhibitor sufficient to provide for variablepropagation distances within said formation prior to gas generation. 30.The method as recited in claim 23 where said blowing agent is containedin said liquid solution in from about 0.05% to about 5.0% by weight. 31.The method as recited in claim 23, where multiple gel plugs and cementplugs are used alternately to exclude production fluids from saidwellbore where said formation has multiple production intervals.
 32. Themethod as recited in claim 23, where said chemical blowing agentdecomposes in the formation thereby releasing a gas which preventsformation fluids from flowing from the formation prior to solidificationof said gel.
 33. The method as recited in claim 23 whereazodicarbonamide is dispersed in a solution of water containing asurfactant.